[bgpd] AS4 bugfix by Chris Caputo <ccaputo@alt.net>

[bgpd] AS4 bugfix by Chris Caputo <ccaputo@alt.net>

* bgpd/bgp_aspath.c: (aspath_make_str_count) "assert (len < str_size)" was
  getting hit under certain 4-byte ASN conditions. New realloc strategy.
* bgpd/bgp_aspath.c: (aspath_key_make) const warning fix.

"%d" -> "%u" 4-byte ASN corrections.  Prevent negative number when ASN is
above 2^31.